Time travel has often been regarded as unattainable, partly due to the notorious “grandfather paradox.” This puzzle speculates what might occur if an individual traveled back in time and stopped their grandfather from having offspring, thereby nullifying the traveler’s own existence. However, a recent investigation may have addressed this dilemma.
By integrating general relativity, quantum physics, and thermodynamics, the research illustrates that time travel could be achievable without leading to these logical inconsistencies.
The mechanics of time loops
Our conventional perception of time is based on Newtonian physics, where events unfold linearly from past to future. Yet, Einstein’s general theory of relativity, which was completed in 1915, challenges this instinctive belief. It shows that the intertwining of space-time can act in ways that contradict common sense, as seen in phenomena such as black holes. One of its most captivating predictions is the possible existence of closed timelike curves — trajectories through space-time that loop back on themselves, theoretically enabling a traveler to revisit the past.
“In general relativity, all varieties of energy and momentum serve as sources of gravity — not solely mass,” study author Lorenzo Gavassino, a physicist at Vanderbilt University, informed LiveScience via email. “This indicates that if matter is in rotation, it can ‘pull’ spacetime along with it. While this effect is minor on planets and stars, what if the entire universe was spinning?”
In a cosmos where all matter rotates, space-time could become so distorted that time effectively curves back on itself, creating a loop. A spacecraft traversing such a loop could, in theory, return to its original location, not only in space but also in time. Although our universe as a whole does not seem to spin in this manner, rotating entities — such as black holes — can create similar results, establishing possible settings for closed timelike curves.
The contradictions of time travel
One of the primary obstacles to time travel is the contradictions it generates. The grandfather paradox is merely one illustration. These dilemmas arise because we assume that the principles of thermodynamics, which dictate heat and energy behavior, would operate normally on a time loop.
“In reality, the law of increasing entropy — a thermodynamic measure of disorder in a system — is the sole physical law that distinguishes between past and future,” Gavassino mentioned. “To our knowledge, entropy is the only reason we remember previous events and cannot foresee future occurrences.”
Entropy influences many aspects of our daily lives, from the aging of our bodies to the way we form memories. Even simple activities, such as walking, depend on friction, which itself increases entropy. So how would these processes operate on a time loop?
A quantum resolution to paradoxes
Gavassino’s study, released on December 12, 2024, in the journal Classical and Quantum Gravity, offers a captivating solution. Drawing from the research of physicist Carlo Rovelli, he illustrated that thermodynamic behavior shifts significantly on a closed timelike curve. In such a loop, quantum fluctuations emerge that can eliminate entropy — a process fundamentally distinct from what we encounter in everyday experience.
These fluctuations could result in substantial impacts on a time traveler. For example, as entropy diminishes, a person’s recollections might disappear, and aging could be reversed. “The increase of entropy is the reason behind our mortality. What occurs when you reverse death?” Gavassino proposed. This phenomenon might even render irreversible actions, like the act of killing one’s grandfather, temporary within a time loop, thus nullifying the paradox entirely.
“Historically, many physicists and philosophers have asserted that if time travel is possible, nature will invariably devise methods to avert contradictory scenarios,” Gavassino remarked. “A ‘self-consistency principle’ was suggested, indicating that all events should align to produce a logically coherent narrative. My research provides the inaugural rigorous derivation of this self-consistency principle directly from established physics. Specifically, I employed the conventional framework of quantum mechanics — without supplementary postulates or contentious assumptions — and showcased that the historical self-consistency naturally emerges from quantum laws.”
Theoretical and practical ramifications
While Gavassino’s conclusions present a persuasive theoretical basis for time travel, the lingering question is: Do closed timelike curves genuinely exist in the actual universe? Most physicists remain doubtful. In 1992, Stephen Hawking notably proposed a “chronology protection conjecture,” suggesting that physical laws could obstruct the formation of time loops altogether. This could entail space-time becoming singular — or collapsing — just prior to the establishment of a loop.
Nevertheless, Gavassino’s contributions are significant in advancing our comprehension.
“What intrigues me about this subject is the manner in which it compels us to contemplate the function of entropy in shaping our perception of the universe, probably my favorite topic in all of physics,” Hawking expressed at that time.
Even if time loops do not exist, analyzing and modeling them could yield insights into genuine phenomena. For instance, investigating how authentic entropy progresses and behaves along a closed trajectory on a subatomic level could uncover fascinating revelations about the characteristics of subatomic systems and their thermodynamics.